A preferred method of lithographic plate-making uses the so-called direct-to-plate systems which allow the direct exposure of the plate precursor instead of having to use a film intermediate as exposure mask. Materials that work according to the diffusion transfer reversal (DTR) process are highly preferred direct-to-plate systems since their high-speed and positive-working imaging process offers the high versatility of being exposed by projection camera's directly from the paste-up copy as well as by most commonly available laser image-setters currently used in graphic arts applications.
The principles of the DTR process have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972). By DTR processing of an information-wise exposed silver halide material, non-developed silver halide present in the photosensitive emulsion layer of the material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a positive silver image having reversed image density values ("DTR image") with respect to the negative black silver image obtained in the exposed areas of the photographic material. In a preferred embodiment, said image receiving element is present in the same material as the photosensitive layer, thereby forming a so-called mono-sheet DTR material, though two-sheets embodiments are also known in the art.
Before processing, a typical DTR lithographic printing plate precursor is characterized by a hydrophilic surface, being water-receptive and non-wettable by ink. After processing, the silver particles that are formed in the image receiving layer at the unexposed areas are hydrophobic (oleophilic) and their ink-receptivity is further enhanced by hydrophobizing agents which are present in the processing liquid(s) so as to form an ink-receptive silver image at the printing areas of the plate.
The printing quality of the lithographic printing plate obtained after processing may be influenced by multiple parameters, e.g. the manufacturing process of the printing plate precursor and of the processing liquids, storage conditions of said precursor and liquids, processing parameters such as temperature, exhaustion of processing liquids due to evaporation, oxidation or non-optimal replenishment, maintenance of the processing apparatus, etc. Once the lithographic printing plate is obtained, its printing quality can not be adjusted anymore and a poor quality can only be remedied by making another plate. It would be advantageous to have a method for verifying the printing quality of the plate before starting the printing job. An even more preferred method should also provide information about the cause of a non-optimal quality by indicating the appropriate adjustments of the plate-making method in an effective and convenient way.